Benefits
Cholesterol and blood pressure
A meta-analysis of 28 trials found whole or ground flaxseed slightly lowered total and LDL cholesterol, while flaxseed oil had no significant effect; a 26-week oil trial found no LDL change. Pooled trials found blood pressure fell under 3 mmHg on average. In a 6-month trial in adults with peripheral artery disease, 30 g a day of milled seed lowered systolic pressure about 10 mmHg versus placebo, mainly in those starting at 140 mmHg or above.
Blood sugar and insulin sensitivity
A meta-analysis of 25 randomized trials found small drops in fasting blood sugar, insulin and insulin resistance with whole flaxseed, but not with flaxseed oil or lignan extract, and no significant change in HbA1c. In adults with prediabetes, 13 g a day of ground seed lowered glucose and insulin versus none, but 26 g did not lower insulin. Flaxseed does not replace diabetes care.
Lignans and menopausal hot flashes
Gut bacteria convert flaxseed lignans into enterolignans, plant compounds thought to have weak estrogen-like effects. In a randomized double-blind trial in 188 postmenopausal women, a daily flaxseed bar providing 410 mg of lignans for 6 weeks did not reduce hot flashes more than a placebo bar (P = 0.29). No study cited here shows a hormonal benefit.
Plant omega-3 (ALA) intake and blood levels
Flaxseed is a rich plant source of ALA, with about 22.8 g per 100 g of seed in USDA data. In controlled trials, eating ground or milled flaxseed every day raised blood ALA levels compared with control foods. ALA from flaxseed oil did not change blood fat levels in a 26-week placebo-controlled trial.
Mechanism of action
ALA intake and blood vessel effects
ALA is the main fat in flaxseed. In the 6-month milled flaxseed trial, blood ALA rose with flaxseed and blood ALA levels correlated with systolic and diastolic pressure. This is a correlation within one trial; how ALA might act on blood vessels is not settled.
Lignan conversion to enterolignans
Flaxseed lignans, mainly secoisolariciresinol diglucoside (SDG), are converted by gut bacteria to enterodiol and enterolactone. Urinary enterolignans rose sharply with a lignan supplement, and blood enterolignans rose with milled flaxseed, where they correlated with changes in diastolic pressure. Lignans are thought to have estrogen-like and anti-estrogen effects, but the hot flash trial was negative.
Fiber and the whole seed versus oil difference
Pooled trials found whole or ground flaxseed lowered LDL cholesterol and blood sugar markers while flaxseed oil did not, which points to parts of the seed other than ALA, such as fiber and lignans. Binding of bile acids by the seed's soluble fiber is a proposed explanation that the trials cited here did not test.
Clinical trials
Meta-analysis of 28 randomized controlled trials of flaxseed, flaxseed oil or lignan supplements in adults, published 1990 to 2008. Flaxseed oil had no significant effect on total or LDL cholesterol, and HDL cholesterol and triglycerides did not change. (Pan et al. 2009, Am J Clin Nutr)
Adults across 28 randomized trials.
All flaxseed forms together lowered total and LDL cholesterol by 0.10 and 0.08 mmol/L. Whole flaxseed lowered them by 0.21 and 0.16 mmol/L and lignan supplements by 0.28 and 0.16 mmol/L. Effects were larger in women, especially postmenopausal women, and in people with high starting cholesterol.
Systematic review and meta-analysis of 15 randomized controlled trials (19 treatment arms) of flaxseed powder, oil or lignan extract. Systolic pressure did not fall significantly with oil or lignan extract, and the effect in trials shorter than 12 weeks was not significant. (Ursoniu et al. 2016, Clin Nutr)
1302 participants across 15 trials.
Flaxseed products lowered systolic blood pressure by 2.85 mmHg and diastolic by 2.39 mmHg versus control. Flaxseed powder lowered both; flaxseed oil lowered diastolic but not systolic pressure; lignan extract lowered neither.
Prospective, double-blind, placebo-controlled randomized trial. Participants ate foods containing 30 g of milled flaxseed or placebo foods every day for 6 months. (Rodriguez-Leyva et al. 2013, Hypertension)
110 adults with peripheral artery disease.
After 6 months, systolic blood pressure was about 10 mmHg and diastolic about 7 mmHg lower with flaxseed than with placebo. In those starting at 140 mmHg or above, systolic pressure fell 15 mmHg. Blood ALA and enterolignans rose 2- to 50-fold with flaxseed, and body weight did not differ between groups.
Double-blind randomized controlled trial. Men and postmenopausal women on a low-fat diet ate baked goods with 40 g/day of ground flaxseed or matching wheat bran products for 10 weeks. Inflammation and oxidative stress markers did not change. NIH-funded. (Bloedon et al. 2008, J Am Coll Nutr)
62 adults with LDL cholesterol of 130 to 200 mg/dL.
Compared with wheat bran, flaxseed lowered LDL cholesterol 13 percent at 5 weeks, but the 7 percent drop at 10 weeks was not significant. At 10 weeks, Lp(a) fell 14 percent and HOMA-IR insulin resistance fell 23.7 percent. In men, HDL fell 16 percent at 5 weeks.
Randomized double-blind trial of 3 g/day of ALA from flaxseed oil capsules versus olive oil placebo capsules for 26 weeks. Changes in LDL, HDL and triglycerides did not differ between groups, and adjusted total cholesterol was higher with flaxseed oil (P = 0.026). (Harper et al. 2006, J Nutr)
56 participants (31 flaxseed oil, 25 olive oil).
Flaxseed oil did not improve any measured blood fat concentration or particle size compared with olive oil after 26 weeks. The authors concluded that ALA from flaxseed oil does not alter these measures.
Systematic review and meta-analysis of 25 randomized placebo-controlled trials (30 treatment arms) of whole flaxseed, flaxseed oil or lignan extract, searched to November 2016. HbA1c did not change, and flaxseed oil and lignan extract showed no significant effects. (Mohammadi-Sartang et al. 2018, Nutr Rev)
Adults across 25 randomized trials.
Flaxseed lowered fasting blood glucose by 2.94 mg/dL, insulin by 7.32 pmol/L and HOMA-IR by 0.49, and raised an insulin sensitivity index. These effects were seen only with whole flaxseed, and the insulin effects only in trials lasting 12 weeks or more.
Randomized crossover trial. Participants ate 0, 13 or 26 g/day of ground flaxseed for 12 weeks each as part of their usual diet. Fructosamine, C-reactive protein, adiponectin and IL-6 did not differ between periods. (Hutchins et al. 2013, Nutr Res)
25 overweight or obese men and postmenopausal women with prediabetes.
Glucose, insulin and HOMA-IR differed by treatment. Glucose fell with 13 g compared with 0 g. Insulin and HOMA-IR fell with 13 g but not with 26 g or 0 g, so the higher amount did not add benefit.
Randomized, double-blind, placebo-controlled crossover trial of 360 mg/day of flaxseed lignan for 12 weeks per period, with an 8-week washout, in people with type 2 diabetes and mildly high cholesterol. Fasting glucose, insulin, insulin resistance and blood lipids did not change significantly. Funded by Chinese government science grants; Frutarom Netherlands and Jarrow Formulas donated the study capsules. (Pan et al. 2007, PLoS One)
68 adults with type 2 diabetes who completed the study.
HbA1c changed by -0.10 percent with the lignan supplement versus +0.09 percent with placebo (P = 0.001), a small difference. Urinary enterolignans rose sharply with the supplement.
Randomized, double-blind, placebo-controlled phase III trial. Women ate one bar a day for 6 weeks holding 7.5 g of flaxseed that provided 410 mg of lignans, or a placebo bar with the same fiber content, and kept daily hot flash diaries. Supported in part by US Public Health Service grants from the National Cancer Institute; Glanbia Nutritionals supplied both bars. (Pruthi et al. 2012, Menopause)
188 postmenopausal women with hot flashes, with or without a history of breast cancer.
Hot flash scores fell by 4.9 with flaxseed and 3.5 with placebo (P = 0.29), a difference that was not significant, and slightly more than a third of women in each group had a 50 percent drop. Both groups reported bloating, gas, diarrhea and nausea, likely from the fiber in both bars. The authors concluded the results do not support flaxseed lignans for hot flashes.